Fix invisible Stripe pay button, add brand favicon/OG image, navbar 3x3-System CTA, blog hero crop

- PaymentStep: bg-brand-primary was never a real Tailwind class, leaving
  the "Jetzt bezahlen" button unstyled/invisible; switched to bg-brand
  with dark text, matching every other CTA on the site.
- New app/icon.svg + app/apple-icon.png + app/favicon.ico: brand-yellow
  rounded square with a bold italic "e", replacing the Next.js default.
- New public/og-image.png (logo on the site's cream background) wired in
  as the SEO fallback default OG image/description/title in payload.ts.
- Navbar CTA: "Klarheitskompass" -> "Mein 3x3-System", now links to
  /3x3-system (page not built yet).
- Blog overview hero image anchored to the top instead of center-cropped.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/checkout/components/PaymentStep.tsx

@@ -80,7 +80,7 @@ function StripePaymentForm({ orderNumber }: { orderNumber: string }) {
       <button
         type="submit"
         disabled={!stripe || submitting}
-        className="rounded-full bg-brand-primary px-6 py-3 text-white font-semibold disabled:opacity-50"
+        className="rounded-full bg-brand px-6 py-3 text-text-primary font-semibold hover:bg-brand-hover active:scale-[0.97] transition-all disabled:opacity-50"
       >
         {submitting ? "Wird bearbeitet…" : "Jetzt bezahlen"}
       </button>